Patch "x86/retpoline: Optimize inline assembler for vmexit_fill_RSB" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

x86/retpoline: Optimize inline assembler for vmexit_fill_RSB

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-retpoline-optimize-inline-assembler-for-vmexit_fill_rsb.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andi Kleen <[email protected]>
Date: Wed, 17 Jan 2018 14:53:28 -0800
Subject: x86/retpoline: Optimize inline assembler for vmexit_fill_RSB

From: Andi Kleen <[email protected]>

commit 3f7d875566d8e79c5e0b2c9a413e91b2c29e0854 upstream.

The generated assembler for the C fill RSB inline asm operations has several issues:

- The C code sets up the loop register, which is then immediately overwritten in __FILL_RETURN_BUFFER with the same value again.

- The C code also passes in the iteration count in another register, which is not used at all.

Remove these two unnecessary operations. Just rely on the single constant passed to the macro for the iterations.

---
 arch/x86/include/asm/nospec-branch.h |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

@@ -183,15 +183,16 @@ extern char __indirect_thunk_end[];
 static inline void vmexit_fill_RSB(void)
 {
 #ifdef CONFIG_RETPOLINE
-	unsigned long loops = RSB_CLEAR_LOOPS / 2;
+	unsigned long loops;
 
 	asm volatile (ALTERNATIVE("jmp 910f",
 				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
 				  X86_FEATURE_RETPOLINE)
 		      "910:"
-		      : "=&r" (loops), ASM_CALL_CONSTRAINT
-		      : "r" (loops) : "memory" );
+		      : "=r" (loops), ASM_CALL_CONSTRAINT
+		      : : "memory" );
 #endif
 }
+
 #endif /* __ASSEMBLY__ */
 #endif /* __NOSPEC_BRANCH_H__ */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-spectre-add-boot-time-option-to-select-spectre-v2-mitigation.patch queue-4.4/x86-retpoline-irq32-convert-assembler-indirect-jumps.patch queue-4.4/x86-retpoline-hyperv-convert-assembler-indirect-jumps.patch queue-4.4/x86-retpoline-entry-convert-entry-assembler-indirect-jumps.patch queue-4.4/kprobes-x86-disable-optimizing-on-the-function-jumps-to-indirect-thunk.patch queue-4.4/x86-retpoline-ftrace-convert-ftrace-assembler-indirect-jumps.patch queue-4.4/x86-retpoline-crypto-convert-crypto-assembler-indirect-jumps.patch queue-4.4/module-add-retpoline-tag-to-vermagic.patch queue-4.4/kprobes-x86-blacklist-indirect-thunk-functions-for-kprobes.patch queue-4.4/x86-retpoline-xen-convert-xen-hypercall-indirect-jumps.patch queue-4.4/x86-retpoline-checksum32-convert-assembler-indirect-jumps.patch queue-4.4/x86-retpoline-fill-return-stack-buffer-on-vmexit.patch queue-4.4/x86-retpoline-add-lfence-to-the-retpoline-rsb-filling-rsb-macros.patch queue-4.4/x86-retpoline-optimize-inline-assembler-for-vmexit_fill_rsb.patch queue-4.4/x86-retpoline-remove-compile-time-warning.patch queue-4.4/retpoline-introduce-start-end-markers-of-indirect-thunk.patch queue-4.4/x86-retpoline-add-initial-retpoline-support.patch